Mr. Saturns made their first appearance in EarthBound. Mr. Saturns are strange creatures that reside in Saturn Valley where they provide health care and hotel stays free of charge, while other towns charge more and more as the game progresses. In EarthBound the Mr. Saturns were captured by Master Belch, one of Giygas's henchmen. Once Master Belch is defeated by Ness and his party, the Mr. Saturn are freed and help Ness on his journey. They are shown to surprisingly be very intelligent as they assist Dr. Andonuts in making the Phase Distorter that allows Ness to travel to the past to fight Giygas. Mr. Saturn have their own eccentric dialect that involves saying either "Zoom", "Boing", "Ding", or the occasional "Dakota" at the end of their sentences. They also speak in a very distinctive font which is based off of Shigesato Itoi's daughter's handwriting at the time. In In an interview with Shigesato Itoi (director of the EarthBound series), Itoi states that Mr. Saturns are the "symbol of innocence". Mother 3 also features Mr. Saturns, who behave the same way as they did in EarthBound. In Mother 3 they are captured by the Pigmask Army where they are being interrogated to tell the location of a Needle, but refuse to tell. Once freed by Lucas and his party they tell the location of the next needle.
In Smash, Mr. Saturns simply walk around until they disappear and can be picked up and thrown at other players. Mr. Saturns having a passive attitude in Smash could be a reference to how they're the "symbol of innocence", and the cry that Mr. Saturns make when thrown at another player is likely a reference to their eccentric dialect in EarthBound and Mother 3. 
Mr. Saturn walks around the stage, pushing other items along with it. It also honks when making contact with an opponent. If thrown, it will deal very little damage and knockback, but it can be used repeatedly, as it does not disappear after one use. As an unconventional meteor smash, Mr. Saturn gives off a large amount of stun when it hits an opponent, making it great for combos. It also does a significant amount of shield damage—21 in Melee and 42 in Brawl. In Smash 4 and Ultimate, Mr. Saturn will always instantly break shields no matter how it is thrown, but can be perfect shielded as with other attacks.
In Brawl, the momentum of a projectile will affect the damage a character will receive; these are recorded in bounces.
In SSB4, damage varies based on distance. Projectiles thrown point-blank do more damage.
Note: A Mr. Saturn cannot be dropped while on the ground; the player will throw it instead.
As an item
Melee marks Mr. Saturn's debut as an item.
The following bonuses are related to Mr. Saturn:
The origins of the Mr. Saturn race are unknown. But many believe that they come from outer space. They are found in Saturn Valley and speak their own unique language. In Super Smash Bros. Melee, Mr. Saturn is just another item with which to pelt foes. These guys are laid back and not into fighting, so you can rest easy... or can you?
As an item
A creature from a race of unknown origin that speaks its own unique language. Some say they came from outer space, but the facts behind their appearance are wrapped in mystery. In Smash Bros., they'll wait a bit after appearing before wandering off on their own. They can be picked up and thrown at foes for minimal damage. Their shield-breaking properties are high.
As an item
In Smash Tour
Mr. Saturn appears as a red item: when selected before a battle, it allows the user's attacks to break shields more easily.
A Mr. Saturn Smash is one of the possible battle types: in it, only Mr. Saturns appear as items.
Mr. Saturns are curious beings that speak their own language. They might not look it, but they're scientific geniuses! In this game, they tend to go for a stroll around the battlefield. Throwing one at an enemy won't do much damage, but Mr. Saturn will take out a shield very easily.
Mr. Saturns are curious beings that speak their own language. They might not look it, but they're scientific geniuses! In this game, they tend to just do their own thing, but you can throw them to deal...a bit of damage. They're good at breaking shields, though, and make adorable noises when they do!
As an item
Mr. Saturn returns as an item again, functioning the same as before. Strangely enough, one is seen being carried by Olimar's Pikmin on both the box and roster art. This is the only item that appears in both of these.
As a costume
Equipping the following Support Spirits will give the player a Mr. Saturn at the start of a match.